Port the tropical tensor network backend to surface code decoder #59

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1. GiggleLiu

   assigned

   ChanceSiyuan

   on Jan 24, 2026
2. ChanceSiyuan commented on Jan 28, 2026

   

   ChanceSiyuan

   on Jan 28, 2026

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   Issue Resolved: Tropical Tensor Network Backend Ported to Surface Code Decoder

   This issue has been resolved with a fully functional tropical TN MAP decoder for surface codes.

   Results Summary

   Metric	Value
   MWPM agreement rate	≥90% on d=3 surface code
   Syndrome satisfaction	100% (valid codewords guaranteed)
   Memory requirement	d=3 works well; d≥5 requires >16GB RAM

   Implementation Evidence

   Core Implementation:

   • tropical_in_new/src/ - Tropical tensor network library:
     • mpe.py - MPE (Most Probable Explanation) inference
     • contraction.py - Tensor network contraction with connected components fix (fix: handle disconnected components in tropical TN contraction #70)
     • tropical_einsum.py - Tropical semiring operations
     • network.py - Factor graph to tensor network conversion

   Surface Code Integration:

   • scripts/analyze_tropical_threshold.py - Complete threshold analysis script that:
     • Builds UAI model from parity check matrix H
     • Runs tropical TN MAP inference
     • Compares with MWPM (pymatching) when available
     • Generates threshold plots
   • bpdecoderplus/dem.py::build_decoding_uai() - UAI model generation from DEM

   Validation Tests:

   • tropical_in_new/tests/test_tropical_mwpm_match.py:
     • test_surface_code_d3_agreement - Verifies ≥90% agreement with MWPM
     • test_surface_code_single_detector_active - Syndrome satisfaction test
     • test_all_factors_included_in_contraction - Verifies contraction tree completeness
   • tropical_in_new/tests/test_connected_components.py - Disconnected component handling

   Output:

   • outputs/tropical_threshold_plot.png - Tropical TN vs MWPM comparison plot

   Integration with bpdecoderplus:
   The tropical decoder uses bpdecoderplus.dem functions (build_parity_check_matrix, build_decoding_uai) with merge_hyperedges=True for efficient computation. The connected components fix (PR #70) ensures all factors are included in contraction.
3. ChanceSiyuan commented on Jan 28, 2026

   

   ChanceSiyuan

   on Jan 28, 2026

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   Closing as resolved - tropical TN backend integrated with surface code decoder, achieving ≥90% MWPM agreement
4. ChanceSiyuan

   closed this as completedon Jan 28, 2026